1) How many deer would you expect in a natural system?
In natural ecosystems the amount of browsing is dependent on the
population level of the indigenous herbivores, which leads to the
question: ‘What is the expected natural population level?’
A standard model used in ecology is that of trophic levels, as first
developed by Lindeman (1942). At the bottom level is the primary
production, i.e. the biomass of plants produced through
photosynthesis. Plants are eaten by the primary consumers
(herbivores) which themselves are eaten by the secondary
consumers (carnivores); in some food chains there can also be
tertiary (and higher) consumers. As a rule of thumb, only 10% of the
energy (biomass = chemical energy) passes up to the next level.
Thus for, example, in a given area, every 100 kg of plant material
produced annually will support 10 kg of herbivores and 1 kg of
Although a generalised and simplified model, this trophic level
approach can be applied to the vegetation of northern Scotland as
shown in Figure 2(a). The data come from various sources. The
plant production figure comes from whole ecosystem production
studies carried out at Moor House National Nature Reserve in the
northern Pennines in the 1970s as part of the International
Biological Programme (Heal et al. 1975). This indicated net annual
plant production averaged over a range of habitats as 635 g dry wt
, range 491-868. This equates to a photosynthetic efficiency of
1%, i.e. 1% of usable solar energy is converted to biomass. The study
site is further south than Scotland, although at 550m altitude has a
similar range of habitats. In Figure 2, a conservative value of 500 g
dry wt m
y-1 has been used.
The dominant indigenous herbivore in the Highlands is the red deer
(Cervus elephas scoticus). Armstrong (1996) gives the daily food
intake of red deer and an average value of 1.75 kg dry matter per
day has here been used. This figure gives a theoretical deer density
of 78 km
assuming 10% of the plant production (biomass) passes
to the primary consumers. This is a simplification in that it assumes
that red deer are the only such consumer, whereas in practice there
will be others dependent on location including roe deer (Capreolus
capreolus), mountain hares (Lepus timidus), voles (Clethrionomys
glareolus), red grouse (Lagopus lagopus scotica) and insects; in many
areas, particularly in the locations of the north and west without
domestic livestock, red deer are the only significant herbivore.
However it does give an order of magnitude indication of the deer
carrying capacity, which in practice will vary depending on the
proportion and palatability of the actual vegetation types present.
The same method indicates a carrying capacity of 131 blackface
sheep, 23 Highland cows or 1,000 mountain hares based on the
food intake data given in Armstrong (1996). The sheep figure
equates well with St Kilda studies where the small Soay sheep in an
unmanaged and unpredated environment have a varying density of
(Clutton-Brock and Pemberton 2004). Additionally,
King and Nicholson (1964) show that in 1952 the densities of free-
ranging sheep on upland farms in Scotland ranged from >25 km
the far north to 167 km
in the south of the area, with the majority
of the area having a density of 50-100 km
The wolf (Canis lupus) figures of 7 km
are added for interest, based
on an average daily meat consumption of 5.53 kg per day
(Glowaciński and Profus (1997), Stahler et al. (2006) indicate a range
2.77-10.4) and 65% water content of the meat.
Figure 2(b) shows how the trophic levels would look, based on
similar assumptions, when deer density is within the range 4-8 km
the level which is recommended for tree regeneration in the
Scottish Highlands (Beaumont et al 1995, Staines 1995, Staines et al.
1995, Milne et al. 1998). It illustrates an unbalanced ecosystem with
a herbivore density such that one square kilometre of land would
not provide enough deer to support a single wolf.
It also indicates that only 1% of the plant productivity is eaten, an
order of magnitude less than would be expected from the
theoretical model. This order of magnitude difference is one reason
why it is so difficult to keep deer numbers low because there is
enough food to support a much higher population; constant culling
is needed to keep the population below the vegetation carrying
capacity. It also indicates that the presence of wolves amongst
unmanaged red deer would not bring the density down to 4-8 km
This trophic level model also assumes that herbivore population
levels are largely determined by food supply as suggested by Milner
et al. (2002): the greater the vegetation productivity, then the greater
the expected animal population. What little research there has been
on this topic shows that unmanaged populations of red deer in
Scotland are somewhere between the theoretical density of 78 deer
and the density of 4-8 km
necessary for woodland survival.
Studies on the Letterewe Estate in the north west of Scotland and
on the Isle of Rum indicate this density to be of the order of 16-18
red deer km
(Milner et al. 2002; Pemberton & Kruuk 2015). The
Letterewe study also indicates that a population level of 16-18 red
would result in 15% offtake of the vegetation biomass, a
higher figure than the 10% of Figure 2, and yet resulting in a deer
population level lower than the 78 km
This discrepancy between the theoretical carrying capacity and the
actual population density can perhaps be explained by the seasonal
nature of plant growth in the Highlands: most primary production
takes place in the relatively short period of late spring and summer.
Grazing at this time will be intense, with a high biomass offtake, as
deer make up for their reserves lost over the winter and also have
to support the current year’s calves. The summer plant production
will support a large number of animals, more than can survive the
lean winter months: as Milner et al. (2002) point out, it is the food
availability during the unfavourable season which will be the
ultimate determinant of herbivore populations and there is little
palatable vegetation available during winter and early spring in the
Highlands. This is reinforced by the conclusions of Pemberton and
Kruuk (2015) who state that in the absence of culling or
supplementary feeding, the population density of red deer on the
island of Rum is strictly dictated by the overwinter carrying capacity
of the land. Therefore it is not only the annual plant production
which will determine the carrying capacity but also how much of
this production the animals can store to maintain their metabolism
through the unfavourable season.
Additionally, the trophic level approach averages the vegetation
production across the whole altitudinal range and landscape, and in
practice significant tracts of land in the Highlands may have a lower
productivity than that modelled, and hence would support a lower
deer population than that modelled. There have been few studies of
the overall vegetation productivity at the landscape scale across the
Scottish Highlands and more research is needed on this topic.
However the key point is that both the trophic level model and
actual studies indicate that the vegetation is supporting a deer
population significantly higher than that necessary to maintain
woodland in the landscape.
Armstrong H. 1996. The grazing behaviour of large
herbivores in the uplands. Information and Advisory Note
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Beaumont D, Dugan D, Evans G, Taylor S. 1995. Deer
management and tree regeneration in the RSPB reserve at
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Clutton-Brock TH, Pemberton J. 2004. Soay sheep: dynamics
and selection in an island population. Cambridge:
Cambridge University Press.
Heal OW, Jones HE, Whitaker JB. 1975. Moor House UK. In:
Rosswall T, Heal OW, editors. Structure and Function of
Tundra Ecosystems. Stockholm: Ecological Bulletin 20, p.
Glowaciński Z, Profus P. 1997. Potential impact of wolves
Canis lupus on prey populations in eastern Poland.
Biological Conservation 80, 99-106.
King J, Nicholson IA. 1964. Grasslands of the forest and sub-
alpine zones. In: Burnett JH, editor. The Vegetation of
Scotland. Edinburgh & London: Oliver and Boyd; p. 168-231.
Lindeman RL. 1942. The trophic-dynamic aspect of ecology.
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Milne JA, Birch JPD, Hester AJ, Armstrong HM, Robertson A.
1998. The impact of vertebrate herbivores on the natural
heritage of the Scottish uplands – a review. Scottish Natural
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Pemberton JM, Kruuk LEB. 2015. Red deer research on the
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Scottish Natural Heritage.
Staines BW. 1995. The Impact of red deer on the
regeneration of native pinewood. In: Aldhous JR, editor. Our
Pinewood Heritage. Edinburgh: Forestry Commission/Royal
Society for the Protection of Birds/Scottish Natural
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deer and their management on the natural heritage of the
uplands. In: Thompson, DBA, Hester AJ, Usher MB, editors.
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Stahler DR, Smith DW, Guernsey DS. 2006. Foraging and
feeding behaviour of the gray wolf (Canis lupus): Lessons
from Yellowstone National Park, Wyoming, USA. Journal of
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WOODLAND OR OPEN GROUND? Scenarios for the persistence of woodland in the presence of grazing
Figure 2a. Trophic level diagram for the Highlands, showing
expected numbers of deer and wolves assuming 10%
biomass transferred upwards to the next trophic level; and
assuming plant primary production of 500 tonnes km
Figure 2b. Trophic level diagram for the Highlands assuming
the same plant productivity and with deer density at 8km
the maximum possible for tree regeneration. This indicates
that there would not be enough deer for a square kilometre
of land to support even one wolf.
78 red deer
4-8 red deer